At the end of S phase, the amount of DNA in the cell would double, since DNA replication occurs during this phase. Therefore, the average DNA content per nucleus would be 16 nanograms.
At the end of G2 phase, the amount of DNA would still be 16 nanograms, since no DNA replication occurs during this phase. However, the cell prepares for mitosis during G2 phase, so the DNA is condensed and organized into chromosomes in preparation for cell division.